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On Osgood theorem in Banach spaces
Authors:Stanislav Shkarin
Abstract:Let X be a real Banach space, ω : [0, +∞) → ? be an increasing continuous function such that ω(0) = 0 and ω(t + s) ≤ ω(t) + ω(s) for all t, s ∈ [0, +∞). According to the infinite dimensional analog of the Osgood theorem if ∫10 (ω(t))?1 dt = ∞, then for any (t0, x0) ∈ ?×X and any continuous map f : ?×XX such that ∥f(t, x) – f(t, y)∥ ≤ ω(∥xy∥) for all t ∈ ?, x, yX, the Cauchy problem (t) = f(t, x(t)), x(t0) = x0 has a unique solution in a neighborhood of t0. We prove that if X has a complemented subspace with an unconditional Schauder basis and ∫10 (ω(t))?1 dt < ∞ then there exists a continuous map f : ? × XX such that ∥f(t, x) – f(t, y)∥ ≤ ω(∥xy∥) for all (t, x, y) ∈ ? × X × X and the Cauchy problem (t) = f(t, x(t)), x(t0) = x0 has no solutions in any interval of the real line.
Keywords:Ordinary differential equations in Banach spaces  Peano theorem  Osgood theorem
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